This invention relates to non-precious metal alloys useful for the manufacture of dental restorations. In particular it relates to the improvement of copper-aluminum alloys with small amounts of zinc.
There can be no doubt that high gold content alloys are the ideal metal for the casting of dental restorations because they have a combination of properties unequaled by any other element. However, as the price of gold has increased in recent years, efforts have been made to reduce the amount of gold used so that dental restorations are affordable to the average person. Reducing the percentage of gold in an alloy is not a satisfactory solution because the desired properties are lost if the amount of gold is reduced to a percentage where the casting might be affordable. Gold is at least ten times as expensive as any nonprecious elements, so there has been great interest in developing alloys made entirely of non-precious metals.
At least as early as 1922 (Hepburn "Notes on Dental Metallurgy"), workers in the dental restoration field have explored aluminum-bronze alloys as a substitution for gold. However, oxidation and galvanic properties encountered in the oral environment usually led to the rejection of these substitutions (Thomson, et al. 50, J. Prosthetic Dentistry, 654 (November 1983)). Specifically the marginal fit of cast copings has not been acceptable in prior art high-copper castings.
While the chemical and physical properties of gold presently to be described are essential to its performance in the mouth under demanding conditions, there is one property that is insisted upon by many consumers: the gold color. Many attempts to emulate the color of gold in a dental alloy have been made. Perhaps the most successful have been copper alloys because the gold color can be achieved with appropriate blending of other metals with copper. Unfortunately, copper alloys have two shortcomings as now formulated. First, the castability is not as good as high gold alloys. Gold alloys are eminently castable, but copper alloys now on the market are not satisfactory. Various elements have been combined with copper in an attempt to improve castability, but the combinations have not been entirely successful.
The second shortcoming of copper alloys for dental restorations has been corrosion. As has been known for centuries, the desired copper color gives way to a green color upon oxidation or corrosion. More importantly, in vivo corrosion results in progressive dissolution of the casting in the oral evironment.
The regulation of dental alloys is highly developed. The standard of comparisons for mechanical properties is ANSI/ADA Spec. No. 5 for Dental Casting Gold Alloy, Type III (hard). Another clinically relevant mechanical property is burnishability (Moon et al., "The Burnishability of Dental Casting Alloys", J. Prosthet. Dent. 36:404-8 (October 1976)).
Corrosion resistance may be compared to existing dental alloys on the market, such as high copper content alloy called "Sybraloy" (Kerr-Sybron), an amalgam alloy certified by the American Dental Association, which has relatively poor corrosion resistance (though clinically acceptable) compared to precious metal alloys. There is on the market lowered gold content Type III casting alloys, such as one called "Minigold" (Williams-Ivoclar), which have relatively poor corrosion resistance for a gold alloy. There is a need for a copper-containing alloy with corrosion resistance superior to known copper-containing alloys like Sybraloy and approaching the corrosion resistance of a precious metal alloy like Minigold.
In addition, it is essential for any dental alloy to be castable for making dental restorations, and non-toxic in the mouth. The latter quality is often measured by the Agar Diffusion Cytotoxicity test to assure an acceptable biological response.
There is a need for a dental alloy having the chemical and physical properties of gold, and with the color of gold, but at a price below the prohibitive cost of gold.
The properties thus sought are:
1. Mechanical properties PA1 2. Biological response PA1 3. Castability PA1 4. Corrosion resistance PA1 5. Gold color PA1 6. Low cost.